Variable insertion sliding post-slotted line tuner having means preventing energy loss past sides of post



1963 w. J. BLEACKLEY 3,110,002

7 .VARIABLE INSERTION SLIDING POST-SLOTTED LINE TUNER HAVING MEANS PREVENTING ENERGY LOSS PAST SIDES OF POST- Filed Feb. 9, 1961 r NV NTOR a. will ATTORNEY 3,3 2 Fatented Nov. 5, l 953 one bad

3,11%),th'32 VARIABLE IN ER'HGN SLIDING PQST-FaLGTTED LENE TUNER HAVING MEANS PREVENTENG ENERGY 19335 PAST @F IGlT William J. Bleackley, Gttawa, Ontario, Qanada, assignor to National Research (Council, @ttawa, fintario, Qanada, a body corporate ct Canada Filed Feb. 9, 1961, Ser. No. 8&ll69 4 Claims. (3. 333-S 8) This invention relates to a variable reactive device, and in particular it relates to a variable reactive device for introducing a variable impedance into a waveguide system.

A waveguide system often contains some type of irregularity which, when the system is in use, causes reflections in the system. For example, bends, twists, joints, discontinuities, couplings or load impedances that are not correctly matched may give rise to reflections. The presence of reflections can usually be said to reduce the efliciency of the system. Also, in many measuring applications, such as those in which a so-called magic T is used to isolate a test piece, reflections introduce unwanted cross coupling and interaction which reduces the accuracy of the measurements. Consequently, it is often desirable to reduce these reflections to as low a value as possible. While good waveguide design will keep reflections to a low value, it is usual to include in a waveguide system some device to tune out unwanted reflections. As is well known, these unwanted reflections can be tuned out by introducing a compensating reflection of the right magnitude and phase.

On the other hand, it is often desirable to be able to introduce a reflection for some specific purpose other than for tuning out an unwanted reflection.

Thus, it is important to be able to introduce a required reflection into a waveguide system. The introduction of a reflection into a waveguide system is, in effect, equivalent to an alteration in the impedance of the waveguide system at that point. In other words, it is important to be able to vary the impedance of a waveguide or to tune a waveguide.

In the past, two well known devices, namely the metal lic post type of reactive device and the E-H tuner, have been used to tune a waveguide.

The metallic post type of reactive device is simply a tuning screw or probe that can be inserted a variable distance into a waveguide in a direction parallel to the electric field to introduce a reflection. Varying the insertion of the screw will vary the magnitude of the reflection, and if the screw is movable along the axis of the waveguide, varying this position will vary the phase of the reflection. Thus, a screw positioned in a slot in the broad side or" a rectangular waveguide where the slot extends along the length of the guide a half a wavelength of the propagated wave, will provide a tuning adjustment. The prior art post type devices generally use two means to prevent radio frequency energy leaking past the post and escaping from the waveguide. The first means consists of metallic spring fingers bridging the gap between the waveguide and the post. Spring fingers are not only expensive but there is a certain amount of radio frequency energy which still leaks past the fingers. There is also a certain amount of wear between the post and the fingers and the contact is not always a steady positive contact. Further, tuning is sometimes erratic because of the aforementioned faults. The second means of preventing leakage employs a choke. When a choke is used with a metallic post device, some of the disadvantages are overcome. However, chokes are expensive and subject to bandwidth limitations. Also there is frequently some radio frequency energy leakage past the chok particularly in the higher frequency bands. Metallic post devices using chokes are usually not satisfactory when Wavelengths in the millimetre region are being handled.

An E-H tuner consists normally of a length of rectangular waveguide at the center of which is placed an E-plane arm arid an H-plane arm. The E-plane arm extends from the top wall of the waveguide and is basically a series tuning element, while the l-i-plane arm extends from the side Wall and is basically a shunt tuning element. Both the arms are adjustable so that any arbitrary impedance can be matched. However, considerable manipulation is sometimes required to match a given impedance. Further, the 5-H tuner is bulky and expensive.

The present invention seeks to overcome the disadvantages of the prior art variable reactive devices by providing a device for introducing a required variable reflection into a waveguide whereby the device is inexpensive, is not subject to excessive wear of critical parts, can reduce leakage or" radio frequency energy to any desired value, is relatively easy to tune, and can be used in han dling wavelengths in the millimetre region.

it is therefore an object of this invention to provide an improved variable reactive device for introducing a variable impedance into a waveguide system.

It is another object of this invention to provide a device for tuning reflections out of a waveguide where the device provides for reducing the leakage of radio frequency energy past the device to any desired value.

it is a further object of this invention to provide an inexpensive and easily operated variable reactive device for introducing a variable impedance into a waveguide.

It is yet another object of this invention to provide a variable reactive device for waveguides that will operate satisfactorily with a waveguide used to propagate waves having a wavelength in the millimetre region.

Further objects and advantages of this invention will appear from the following description taken in conjunction with the drawings in which:

FIGURE 1 is a side View of a section of waveguide incorporating the variable reactive device of one form of this invention with parts broken away to provide a clearer picture,

FIGURE 2 is an end view of the waveguide and device of FIGURE 1,

FIGURE 3 is an isometric view of a section of a waveguide incorporating the variable reactive device of this invention in another form, and

FIGURE 4 is a sectional end view of a waveguide and a variable reactive device according to another embodiment of the invention.

Briefly, the invention is a variable reactive device generally of the metallic post type but without requiring the spring finger arrangements or thechoke arrangements of prior art devices. The invention comprises a first waveguide which propagates a required wave or waves with a second waveguide joining it. A rod of dielectric material is inside the second waveguide and in slidable contact with the sides. A metallic post is embedded in the end of the rod so that the rod with post can be advanced and the post inserted into the first Waveguide by a variable amount to introduce a variable impedance into the first waveguide. The dimensions of the second waveguide are such that its cut-cit wavelength is much less than the wavelength of the waves being propagated and leakage of radio frequency energy past the post along the second waveguide is avoided.

Referring now to FIGURES l and 2, a portion or section of a waveguide 113 of rectangular configuration is shown. The waveguide ill is designed to propagate waves in a given frequency range and in a desired mode. In this description only the dominant mode is considered, although it is well known that other modes can be propae al gated and the invention could be used with a waveguide system propagating other modes.

A waveguide 11 of circular configuration is joined to waveguide iii. A rod 12 is contained in wa eguide and is slidably movable Within waveguide ll. in the embodiment of FIGURES l and 2, the rod 12 is shown as having substantially the same cross-section as the inside of waveguide 11 so that rod 32 slidably contacts the inner surface of waveguide 11. This is structurally convenient. It is, however, not necessary that the rod 12 have the same cross-section as the inside of wavegui e ll as long as .the rod 12 is slidably supported within waveguide ll.

The rod 12 is preferably made of a low loss material with a low dielectric constant. Examples of suitable materials for rod 12 are polystyrene and Teflon. A metallic post 14, preferably made of a good conducting material such as copper, silver, silver plated brass, etc., is embedded in one end of rod 12, and, while only a portion of post i4 is shown embedded, it is understood that the post 14 could be completely embedded so that the end of post 14 would be flush with the end of rod 1'2. In the preferred embodiment, however, the end portion only of post 14 is embedded so that when the post 14 is advanced to its farthest position (projecting into waveguide 10 as far as possible) the rod 12 does not enter the waveguide All. This reduces the dielectric losses associated with the device.

As is well known, for a given waveguide configura'don with [given dimensions and for a given mode, there is a cut-oil frequency (and associated cut-off wavelength) for propagation through the given waveguide. Waves having frequencies less than the cut-E frequency (or waves having a wavelength longer than the cut-01 wavelength) will not propagate through the waveguide but are rapidly attenuated. In order to simplify the description, the waveguide 11 may be considered as having two portions. The first portion, designated 11a, extends from the inner surface of the top Wall of waveguide 16 up to a plane passing through waveguide 11 at the top end of post 14-, and may be referred to as a coaxial waveguide portion. The second portion of waveguide 11 is designated as 11]; and extends from the plane passing through waveguide ill at the top end of post 14- upwards to the end of waveguide ll. The junction point or plane of waveguide portions 11a and 11b constitutes a discontinuity in waveguide ll. In the present invention, the dimensions of waveguide portion 11b are made such that its out-oil wavelength is much less than the wavelength of the wave being propagated through waveguide 1t? and which will appear also in waveguide portion 11a. Consequently any radio frequency energy passing post 14 and entering the waveguide 111: is almost totally reflected at the junction of waveguide portions 11a and 1119 because of the discontinuity there. The small amount of radio frequency energy passing the junction into waveguide portion 11b is rapidly attenuated. The attenuation is proportional to the length of waveguide portion lib. By increasing the length of waveguide ll, that is by increasing waveguide portion 11b, any desired amount of attenuation of the leakage energy can be achieved.

While FIGURES l and 2 show waveguides it and "ii as rectangular and circular waveguides, respectively, it will be understood that any waveguide cross-section may be used for either waveguide. It is, however, convenient to have waveguide 11 of circular cross-section for practical structural and manufacturing purposes. Regardless of waveguide configurations, the waveguide portion 11b must have dimensions such that its cut-0d wavelength is much less than the wavelength of the wave propagating through guide ltl.

it will be apparent that one variable reactive device, as shown in FIGURES l and 2, that is not movable along the length of the waveguide it cannot be used to insert reflections of all phases. Consequently, for complete control of reflections, a plurality of variable reactive 2- devices are required. it is well known that four metallic post type reactive devices spaced one-eighth wavelength apart would be capable of inserting any possible reflection, and three such devices will include coverage of almost all the possible reflections. Consequently, in order to tune out all possible unwanted reflections from a waveguide, that is, in order to be able to insert any reflection, four variable reactive devices according to the embodiment of FIGURES l and 2 would be re uired. If such complete coverage of possible reflections is not necessary, three, two, or even one such devices might be adequate.

As previously mentioned, the rod 12 does not have to have the same cross-section as the inside of waveguide 11. In FIGURE 4, an embodiment is shown where the rod 12 is centrally supported in waveguide 11 by a plurality of spacers 29' (only one of which is shown). The rod 12 has a metallic post 14 afiixed to one of its ends by any convenient means. The rod 12. is slidably supported by spacers 29 so that the post 14 can be inserted into waveguide lll by a variable amount. The FIGURE 4 embodiment functions in the same manner as the embodiment of FlGURES l arid 2 and further description is believed unnecessary.

it is perhaps more convenient to use the variable reactive device shown in FIGURE 3 to introduce a variable impedance into a waveguide system. In FIGURE 3, where the same numbers designate the same components as previously, waveguide ltl is again shown of rectangular configuration. A waveguide ii is carried by arms 15 and 16 which slidably engage longitudinal rail member 17 and by arm 18 which slidably engages longitudinal rail member 1/. The rail members 17 and 19 are parallel and supported at their ends by supports 2 and 25 which are fixed to Waveguide it The rail members 17 and 19 serve to support and guide waveguide 11' as it is moved along waveguide it The waveguide ll terminates at the end adjacent waveguide it) in an end plate 21. As the waveguide 11 is moved between rail members It? and .19, the end plate 21 remains adjacent the upper wall of waveguide Ill. A rod 12 of dielectric material is slidably movable within waveguide ill As before, it is convenient to have rod 12 of substantially the same cross-section as the inside of waveguide ll so that the rod 12 makes direct sliding contact with the waveguide 11'. A metallic post 14 is embedded in or atlixed to the end of rod 12. A slot 22 in the upper wall of waveguide it permits the metallic post 14 to be inserted into waveguide 16 at any point along the length of the slot 22. A scale 23 on a side wall of waveguide 163 provides a convenient reference for positioning waveguide 11a along the slot 22.

Thus, the metallic post 14 may be inserted into waveguide ltl through slot 22 by a variable amount by moving the end of rod 12 remote from post While rod 12 has been shown in the drawings with a free or remote end by which it can be adjusted and no mechanical adjustment means has been shown, it is of course known that various mechanical means may be used to achieve a more accurate control over the position of rod 12 and post 14. For example, vernier and micrometer adjus ment means for waveguide tuning devices are well known in the art. The metallic post 14 may also be moved along the length of slot 22 by moving waveguide 11 with its arms l5, l6 and 18-, along rail members 3.7 and if the post 14 can be varied in its longitudinal position through half a wavelength of the wave being propagated, then a refl ction of any phase may be introduced into waveguide iii. The amount of insertion of post 14 with rod 12 will govern the magnitude of the reflection.

As previously set forth, extending the length of waveguide ll' will reduce leakage of radio frequency energy through waveguide ii to any required value.

It will be seen that there are no great problems in using this reactive device with waveguides propagating waves in the millimetre wavelength region.

Radio frequency energy may readily pass spring fingers and possibly a choke in the millimetre region because of mechanical difficulties with small dimensions and tolerances. However, the attenuating waveguide 11, or as it may be termed the beyond cut-ofi waveguide, will operate quite well at these wavelengths and is quite effective at reducing leakage of radio frequency energy.

In the operation of a variable reactive device according to this invention, there may arise a situation where the metallic post 14 is approximately one half wavelength long and the post 14 becomes resonant. Should this situation arise, the length of metallic post 14- should be altered. It may be convenient to provide the rod 12 with a metallic post embedded in each end with each post of a different length. A reversal of the rod to substitute one post for the other at the critical frequency will avoid having the post resonate.

From the foregoing description it will be seen that the invention provides a simple, inexpensive variable reactive device for introducin a variable impedance into a waveguide that reduces radio frequency leakage to a low value and that can be used in the millimetre wavelength region.

While particular embodiments of this invention have been disclosed, modifications and variations will occur to those skilled in the art, and it is desired to include all such modifications and variations that are within the true spirit and scope of the invention.

I claim:

1. A variable reactive device for introducing a variable impedance into a first waveguide formed by a first tubular conducting Wall and designed to propagate Waves in a given frequency range comprising a second tubular conducting wall forming a second waveguide joining said first wall,

a rod or low loss insulating material sl-idably mounted within said second wall,

a metallic post fixed to an end portion of said rod and having a terminating end in engagement with said rod,

said rod and said post being longitudinally movable within said second wall to insert said post a variable amount into said first waveguide to introduce a variable impedance therein,

said post having a portion disposed within said second wall and constituting therewith a coaxial section of said second waveguide disposed to one side of and terminating in a plane defined by said end of said post,

said second waveguide having a remaining section on the other side of said plane with a cut-oil frequency that is higher than the frequencies in said given range,

the said plane at the junction of said coaxial section and said remaining section of said second waveguide constituting a reflective discontinuity for waves of said given frequency range in said coaxial section.

2. A variable reactive device for introducing a variable impedance into a first waveguide formed by a first tubular conducting wall and designed to propagate waves in a given frequency range comprising a second tubular conducting wall of circular crosssection "forming a second waveguide joining said first wall,

a rod of low loss dielectric material having a circular cross-section substantially the same as said second wall within said second wall and slidably engaging the inner surface of said second wall,

a metallic post fixed to and extending into an end portion of said rod and having a terminating end within said rod,

said rod with said post being longitudinally movable within said second wall to insert said post a variable amount into said first Waveguide to introduce therein a variable impedance,

said post having a portion disposed within said second wall and constituting therewith a coaxial section of said second waveguide disposed to one side of and terminating in a plane defined by said end of said post,

said second Waveguide having a remaining section on the other side of said plane with a cutolT frequency that is higher than the frequencies in said given range,

the said plane at the junction of said coaxial section and said remaining section of said second waveguide constituting a reflective discontinuity for waves of said given frequency range in said coaxial section.

3. A variable reactive device for introducing a variable impedance into a first waveguide formed by a first tubular conducting wall of rectangular cross-section designed to propagate waves in a given frequency range comprising a second tubular conducting wall of circular crosssection forming a second waveguide joining said first wall,

a rod of low dielectric constant material having a circular cross-section substantially the same as said second wall mounted within said second wall slidably engaging the inner surface of said second wall,

a metallic post axially aligned with said rod extending into an end portion thereof and being fixed thereto,

said post having a terminating end within said rod,

said rod with said post being longitudinally movable within said second Wall to insert said post a variable arnount into said first waveguide to introduce therein a variable impedance,

said post having a portion disposed within said second wall and constituting therewith a coaxial section of said second waveguide disposed to one side of and terminating in a plane defined by said end of said post,

said second waveguide having a remaining section on the other side of said plane with a cut-off frequency that is higher than the frequencies in said given range,

the said plane at the junction of said coaxial section and said remaining section of said second waveguide constituting a reflective discontinuity for waves of said given frequency range in said coaxial section.

4. A variable reactive device for introducing a variable impedance into a first waveguide formed by a first tubular conducting wall of rectangular cross-section designed to propagate waves of a given frequency and having in said wall a slot ex-tending lengthwise of said wave guide substantially one half wavelength at said given frequency comprising a second tubular conducting wall of circular crosssection forming a second waveguide,

means movably mounting said second wall abutting against said first wall with the axis of said second wall passing through the center of said slot and guiding said second wall along the length of said slot,

a rod of low dielectric constant material having a circular cross-section substantially the same as said second wall within said second wall slid-ably engaging the inner surface of said second wall,

a metallic post axially aligned with said rod extending into an end portion thereof and being fixed therein,

said post having a terminating end within said rod,

said rod with said post being longitudinally movab-le within said second wall to insert said post through said slot a variable amount into said first Waveguide to introduce therein a variable impedance,

the position of said post along the length of said slot and the amount of insertion of said post into said first Waveguide determining the value of the variable impedance introduced,

said post having a portion disposed Within said second wall and constituting therewith a coaxial section 0 said second Waveguide disposed to one side of and terminating in a plane defined by the said end of said post,

said second Waveguide having a remaining section on the other side of said plane with a cut-off frequency that is higher than said given frequency,

the said plane at the junction of said coaxial section and said remaining section of said second Waveguide constituting a reflective discontinuity for Waves of said given frequency in said coaxial section.

References Qited in the file of this patent UNITED STATES PATENTS King Apr. 16, 1940 Learned Feb. 23, 1954 Colin Oct. 1, 1957 Gilchrist Dec. 2, 1958 Murphy Mar. 31, 1959 Hessler Oct. 20, 1959 Brockwell Mar. 21, 1961 Christensen et a1. Aug. 15, 1961 OTHER REFERENCES Technique of Microwave Measurements, vol. 11 of Radiation Laboratory Series, pages 478 to 483 cited, TK 

1. A VARIABLE REACTIVE DEVICE FOR INTRODUCING A VARIABLE IMPEDANCE INTO A FIRST WAVEGUIDE FORMED BY A FIRST TUBULAR CONDUCTING WALL AND DESIGNED TO PROPAGATE WAVES IN A GIVEN FREQUENCY RANGE COMPRISING A SECOND TUBULAR CONDUCTING WALL FORMING A SECOND WAVEGUIDE JOINING SAID FIRST WALL, A ROD OF LOW LOSS INSULATING MATERIAL SLIDABLY MOUNTED WITHIN SAID SECOND WALL, A METALLIC POST FIXED TO AN END PORTION OF SAID ROD AND HAVING A TERMINATING END IN ENGAGEMENT WITH SAID ROD, SAID ROD AND SAID POST BEING LONGITUDINALLY MOVABLE WITHIN SAID SECOND WALL TO INSERT SAID POST A VARIABLE AMOUNT INTO SAID FIRST WAVEGUIDE TO INTRODUCE A VARIABLE IMPEDANCE THEREIN, SAID POST HAVING A PORTION DISPOSED WITHIN SAID SECOND WALL AND CONSTITUTING THEREWITH A COAXIAL SECTION OF SAID SECOND WAVEGUIDE DISPOSED TO ONE SIDE OF AND TERMINATING IN A PLANE DEFINED BY SAID END OF SAID POST, SAID SECOND WAVEGUIDE HAVING A REMAINING SECTION ON THE OTHER SIDE OF SAID PLANE WITH A CUT-OFF FREQUENCY THAT IS HIGHER THAN THE FREQUENCIES IN SAID GIVEN RANGE, THE SAID PLANE AT THE JUNCTION OF SAID COAXIAL SECTION AND SAID REMAINING SECTION OF SAID SECOND WAVEGUIDE CONSTITUTING A REFLECTIVE DISCONTINUITY FOR WAVES OF SAID GIVEN FREQUENCY RANGE IN SAID COAXIAL SECTION. 